No matter how you slice it, nine months seems like an awful long time to spend cooped up in the womb, even if it is "biologically essential" or "how mammals work." We already know that fetuses kick, stretch, swallow, and even hiccup to pass the time, but sometimes they've just got to yawn.

The photo on the left actually shows a fetus mid-yawn. It was taken as part of a study by researchers at Britain's Durham and Lancaster Universities, who took a series of 4D scans of healthy fetuses to work out when they were yawning and when they were simply opening their mouths. Based on the researchers' criteria, the fetuses yawned about half of the times they opened their mouths, and their yawning seemed to drop off after about the 28th week of gestation.

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Exactly why the fetuses yawn is unknown — hell, we're still not entirely sure why we yawn. But this would seem to suggest that the reflex has some connection to fetal development, and in future our ability to track yawning in the womb could provide another way to gauge the health of the fetus. In a statement, lead researcher Dr. Nadja Reissland added:

"The results of this study demonstrate that yawning can be observed in healthy fetuses and extends previous work on fetal yawning. Our longitudinal study shows that yawning declines with increasing fetal age. Unlike us, fetuses do not yawn contagiously, nor do they yawn because they are sleepy. Instead, the frequency of yawning in the womb may be linked to the maturing of the brain early in gestation. Given that the frequency of yawning in our sample of healthy fetuses declined from 28 weeks to 36 weeks gestation, it seems to suggest that yawning and simple mouth opening have this maturational function early in gestation."